Escaping vapors of volatile fuel and chemical fluids in mobile and stationary storage tanks is not only a big loss of raw materials, but also a major source of air pollution, fire and explosion hazards, etc.
In the case of automobile engines without evaporative emission control, almost 20% of the gasoline is lost to the atmosphere by way of vaporization and breathing in the fuel storage tank and the carburetor bowl. At high temperature or low ambient pressure at high elevation, a larger percentage of vapors in the fuel tank and carburetor bowl goes out into the atmosphere through the external vents. When fuel is withdrawn from the tank while the engine is running, or when the vapors condense as the temperature drops, fresh air comes in through the vents to take up the vacated space, and the moisture-laden air may form ice in the fuel system at low temperature and even change the characteristics of the fluid, such as alcohol, which is readily miscible with water.
Since 1971, all motor vehicles in the U.S. are required by law to have a vapor recovery system. It consists of carbon canisters, liquid-vapor separators, vent and purging lines, etc.. When the engine stops, vapors in the fuel tank and the carburetor bowl are absorbed by the activated carbon in the canister. As soon as the engine is started, the vacuum in the manifolds draws in fresh air through the carbon canister and the vapor is purged into the engine for burning. The system is complicated and needs frequent cleaning or replacing of the air filter, the activated carbon, etc.. Besides, exchange of vapors and external air still takes place in the carbon canister.
The petroleum and chemical industries, in an effort to combat evaporative or boil-off loss of raw materials and leaking of deadly toxic fumes from giant storage tanks above or under the ground, have come up with various proprietary designs. The widely used method is the installation of a floating roof inside the vented fixed roof of the storage tank. A giant metal pan or deck sometimes aided by floats and pontoons, rides freely on the surface of the liquid, and the annular space between the floating roof and the tank shell is sealed by means of sliding mechanical shoes, flexible elastomeric scuffbands, foam-filled seals, etc. While the floating roof greatly reduces the area of surface of the fluid exposed to the atmosphere, exchange of fuel vapors and external air still takes place in the sliding contact seals. While the system may be useful for fluids having low vapor pressure in stationary storage tanks, it is not effective for stopping vapor emission of fluids having high vapor pressures in a storage tank constantly in unsteady motion, wherein sloshing and surging of the fluid, and heaving, rolling and pitching of the free-floating deck can quickly destroy the effectiveness of the sliding contact sealing.
During the refueling operation at a filling station, gasoline from the filler nozzle splashes down to the fluid in a conventional fuel tank and the rising fluid drives the fumes out into the atmosphere. In order to combat the air pollution at busy filling stations, efforts have been made in modifying the dispensing units and the nozzles or using a double-walled hose in which one passage is used for fuel supply and the other for drawing the vapors in the fuel tank into the station's underground storage tank.
In recent years, leaking of deadly toxic fumes and liquids from chemical tanks into the atmosphere and underground water not only becomes a serious threat to human health and environmental quality, but also hinders the rapid development of high technology industries.